A starter of said type comprises a support, an electric motor which is arranged on the support and which serves for driving a pinion in rotation, and a solenoid drive which is arranged on the support and which serves for the axial adjustment of the pinion between an active position, which is provided for the drive of a gearwheel of the internal combustion engine, and a passive position, which is axially offset with respect to the active position. The solenoid drive comprises a plunger stop which is static with respect to the support, a plunger which is axially adjustable relative to the plunger stop, and a cylindrical coil arrangement which is arranged on the plunger stop and which surrounds a cylindrical coil interior of the coil arrangement in a circumferential direction. Furthermore, the plunger stop has a cylindrical section which projects axially into the coil interior.
For the starting of the internal combustion engine, the solenoid drive is activated so as to transfer the pinion of the electric motor from the passive position into the active position. In the active position, the pinion meshes with a gearwheel of the internal combustion engine, which may be formed for example on a flywheel of a drivetrain of the internal combustion engine. The electric motor then drives the pinion, which in turn drives said gearwheel, whereby a crankshaft of the internal combustion engine is set in rotation in order to start the internal combustion engine. When the internal combustion engine has started and its crankshaft is driven by reciprocating movements of the pistons of the internal combustion engine, the solenoid drive is operated such that the pinion is returned from the active position into the passive position. In the passive position, the pinion disengages from said gearwheel, that is to say no longer meshes with the latter.
To be able to adjust the pinion from the passive position into the active position and to be able to hold the pinion fixed in the active position, the coil arrangement must provide relatively large magnetomotive force in order to draw the plunger into the coil interior, and hold it there, for the active position. Since, for the purposes of a failsafe design, the plunger is preferably drawn into the coil interior counter to the action of a restoring spring, relatively high magnetic forces are required in particular to hold the plunger static in the active position of the pinion, such that the coil arrangement is supplied with a correspondingly high level of electrical power.
The pinion normally has a circumferential toothing with axially extending teeth. Complementary with respect to this, the gearwheel of the internal combustion engine likewise has a circumferential toothing with axially running teeth. Upon a transfer of the pinion from the passive position into the active position, the teeth of the pinion engage into tooth spaces of the gearwheel. However, in many situations, axially leading tooth flanks of the teeth of the pinion do not pass directly into the tooth spaces of the toothing of the gearwheel but strike axial tooth flanks of the teeth of the gearwheel. In order that the teeth of the pinion nevertheless find their way into the tooth spaces of the gearwheel and can engage therein, the electric motor of the starter may be actuated so as to effect a rotation of the pinion already during the adjustment of the pinion from the passive position into the active position. Said rotation for the threading-in of the pinion into the gearwheel is expediently performed with a considerably reduced torque and/or with a considerably reduced rotational speed in relation to the subsequent starting process, when the pinion is fully engaged with the gearwheel.
Owing to the relatively high magnetic force with which the plunger is drawn into the coil interior, as described above, the pinion may, by way of its axially leading tooth flanks, collide with the opposite axial tooth flanks of the gearwheel with corresponding intensity, increasing the wear of the toothings of pinion and gearwheel. Furthermore, the toothings may bear against one another by way of the axial tooth flanks with a relatively high force, whereby a correspondingly high level of friction must be overcome in order to rotate the pinion relative to the gearwheel such that the toothing of the pinion can mesh with the toothing of the gearwheel. As a result, there is the risk of increased wear here too.
A generic starter is known for example from U.S. Pat. No. 8,421,565 B2. To solve the abovementioned problem, in the case of a known starter, said document proposes a complex construction of the coil arrangement within the solenoid drive, wherein a retraction coil for pulling the plunger into the coil interior and a holding coil for holding the plunger that has been pulled into the coil interior are arranged axially separately from one another. It is also proposed that the plunger be equipped, on its outer circumference, with an encircling groove which, in the passive position, is situated radially opposite an edge region circumferentially surrounding a passage opening, through which the plunger extends axially, of a face side wall of a solenoid housing. In this way, in the passive position, there is a radial gap between plunger and edge region. As the plunger is retracted into the coil interior, the circumferential groove moves into the coil interior and thereby departs from the abovementioned edge region of the face side wall, such that said edge region is subsequently situated radially opposite a plunger longitudinal section adjoining the circumferential groove. As the plunger is retracted, therefore, a radial spacing between said edge region and an outer side of the plunger is varied, specifically reduced, whereby the density of the magnetic field lines transmitted from said edge region to the plunger when the coil arrangement is activated is varied, specifically increased. The density of the magnetic field lines however correlates with the acting magnetic forces. The circumferential groove formed on the plunger thus yields a reduction in the acting magnetic forces at the start of the retraction movement of the plunger when the pinion is to be transferred from the passive position into the active position. The known measures are however relatively cumbersome to realize. Furthermore, the attractive force that pulls the plunger into the coil interior is reduced only to a relatively small extent by the annular groove, as said annular groove ultimately merely effects a deflection of the field lines. Also, the annular groove is maintained and, even when the plunger has been retracted into the coil interior, causes a deflection of the field lines in the plunger, thus reducing the attainable magnetic forces.